Redox amplification processes have been described, for example in British Specifications Nos. 1,268,126; 1,399,481; 1,403,418; and 1,560,572. In such processes color materials are developed to produce a silver image(which may contain only small amounts of silver) and treated with a redox amplifying solution (or a combined developer/amplifier) to form a dye image.
The developer/amplifier solution contains a color developing agent and an oxidizing agent which will oxidize the color developing agent in the presence of the silver image which acts as a catalyst.
Oxidized color developer reacts with a color coupler to form the dye image. The amount of dye formed depends on the time of treatment or the availability of the color coupler and is less dependent on the amount of silver in the image than is the case in conventional color development processes.
Examples of suitable oxidizing agents include peroxy compounds including hydrogen peroxide and compounds which provide hydrogen peroxide, e.g., addition compounds of hydrogen peroxide such as perborates and addition compounds of hydrogen peroxide with urea. Other oxidizing agents include cobalt (III) complexes including cobalt hexammine complexes; and periodates. Mixtures of such compounds can also be used.
In color photography development (whether redox or conventional) it is necessary at an appropriate stage to remove the silver image which, if left behind would darken the dye image. Also it is necessary to remove unused silver halide because it darkens on exposure to light. To remove the silver, the practice has been to convert the silver to silver halide with a suitable oxidizing agent known in the art as a bleach and then remove the silver halide with a fixing agent.
Suitable oxidizing agents are potassium ferricyanide, or ferric iron complexed with ethylene diamine tetraacetic acid acting in the presence of potassium bromide. The two steps may be combined using a solution called a bleach-fix or blix.
In the case of a bleach-fix employed after a redox amplification, the solution only needs small amounts of iron (III) and thiosulfate because there is usually only a small amount of silver to remove.
U.S. Pat. No. 5,200,301 relates to a color photographic silver halide material which is particularly suitable for colored machine-readable identification systems. Machine-readable detection systems normally operate by I.R. absorption of the data applied and this patent is concerned with the problem of providing a color photographic material which produces a clear brilliant dye image and also shows adequate I.R. absorption in the data part, the I.R. zones being photographically produced. Although directed to a different problem from that with which the present invention is concerned, a process for the redox development of photographic materials containing greater than 200 mg/m.sup.2 of silver is described in which the development stage is faster than conventional. A subsequent fixing step is disclosed but there is no bleaching step with the result that the silver is left in the developed photographic material.
Redox development/amplification is intended primarily for photographic materials containing low amounts of silver, especially for low silver photographic papers, i.e., containing less than 200 mg/m.sup.2 of silver. The time of development using a redox system with low silver photographic paper is broadly similar to that for a conventional development process applied to conventional, i.e., higher silver-containing materials.
However, it is very desirable to be able to reduce the development time because this enables more photographic material to be processed with existing processing equipment or for the processing equipment to be made smaller.